A technology will be developed for the production of miniature biosensors that can be used for in vivo and in vitro monitoring of low molecular weight analytes (less than 1000) such as metabolites, drugs, hormones, and toxic substances. The technology is based on fiber optics and immunoassay principles, i.e., a receptor (e.g., antibody) is selected for the drug (e.g., hapten), and the quenching of a fluorescently labelled analog of the drug is used to measure the extent of binding between receptor and drug. While immunoassays are discrete tests, the biosensor technology being developed provides continuous on-line monitoring. Here, the antibody (or its equivalent) and a high MW fluorescent analog are retained in a selectively porour 'micro test tube,' formed by a hollow dialysis fiber (about 300 microns diameter). The biosensor readily responds to external concentration changes of analyte due to the reversible displacement of the fluorescent analog from the receptor. Fluorescent intensity is monitored with a singleoptical fiber inserted into the hollow dialysis fiber. The use of fiber optics provides the capability for a wide variety of optical measurements, e.g., colorimetry, fluorescence, light scattering, optical rotation, to measure the extent of binding. This technology has the potential to convert the existing immense repertoire of immunoassays into a multitude of specific continuous miniature biosensors. Eventually, as miniature optic electronic devices (solid state lasers, photodetectors, and couplers) become available, implantable biosensors will be practical. Applications for these biosensors include: continuous drug monitoring in intensive care, e.g., gentamicin, methotrexate; feedback control systems, e.g., glucose monitoring for the artificial pancreas; and the measurement of toxic chemicals in the environment - all multi-million dollar businesses.